Determinants of response to hypercapnia and to progressive and transient hypoxia in obstructive sleep apnea patients.

Abstract

The knowledge regarding the control of breathing during wakefulness in patients affected by obstructive sleep apnea (OSAS) is still challenging. The aim of this study is firstly to analyze hypoxic and hypercapnic ventilatory response in OSA patients in comparison to controls and secondly, to investigate correlations between chemosensivity and both lung function tests, such as arterial blood gas analysis and spirometric parameters, and clinical characteristics, such as age and BMI. Seventeen never treated OSA patients (16M; 53±13.2 years; BMI=34.5±8.1; AHI=45±14.7) underwent nocturnal cardiopulmonary monitoring test, complete lung function tests (spirometry, lung volumes and arterial blood gas analysis on room air). Read's rebreathing test was used to evaluate hypercapnic ventilatory response (HVR<inf>CO2</inf>); hypoxic ventilatory response (HVR<inf>O2</inf>) was studied through progressive and transient methods. The response was expressed in terms of slope of linear regression for HVR<inf>CO2</inf> and of hyperbolic curve for HVR<inf>O2</inf> between minute ventilation (VE) or mean inspiratory flow (VT/Ti) and PetCO<inf>2</inf> or PetO<inf>2</inf>. The OSA group showed increased transient, but not progressive, HVR<inf>O2</inf> and a lower HVR<inf>CO2</inf>. A lower HVR<inf>CO2</inf> was predicted by greater values of BMI (P<0.01). Progressive HVR<inf>O2</inf> increased with age and lower FEV<inf>1</inf>/VC, while transient HVR<inf>O2</inf> (P<0.05) was inversely correlated with the nocturnal lowest SaO<inf>2</inf> (P<0.01). Nocturnal intermittent hypoxic stimuli increases daytime glomic reactivity to transient hypoxia, while BMI is the main independent determinants of the bulbar response to hypercapnia in normocapnic OSAS patients.